Sound silencing method and apparatus

ABSTRACT

This disclosure deals with a novel sound muffler for gas turbine engines and the like employing in the same duct section both longitudinal baffles and outwardly transversely extending sidebranch cavities.

[ June 12, 1973 2,051,515 8/1936 Bourne 181/42 6/1961 181/59 SOUND SILENCING METHOD AND 2,989,136 Wohlberg APPARATUS [75] Inventors: Istvsin L. Vr, Arlington; Ulrich J.

my .m n l mm UC AG 0 6 9 1 2 1 4 0 5 9 1 FOREIGN PATENTS OR APPLICATIONS Kurze, W. Roxbury, both of Mass. 3 19 3 Bolt Beranek and Newman, Inc., Cambridge, Mass.

Dec. 13, 1971 1,200,007 9/1965 Germany [73] Assignee:

733,329 Great Britain 783,055 9/1957 Great Britain [22] Filed:

Appl' 207008 Primary Examiner-Richard B. Wilkinson Assistant Examiner-John F. Gonzales T C A R T S s m A m & S 6 .m J V; e m

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This disclosure deals with a novel sound muffler for 181/48, 59,35 R, 33 HB, 33 HE gas turbine engines and the like employing in the same duct section both longitudinal baffles and outward] versely extending side-branch cavities.

y trans- [5 6] References Cited UNITED STATES PATENTS 3,353,626 11/1967 Cremer et 181/48 6 Claims, 4 Drawing Figures GAS FLOW PAIENIEB JUN 1 2 ms sum 1 or z ll I GAS FLOW ISTVAN L ULRICH J. KURZE, INVENTORS Q MMM ATTORNEYS PATENIEU Jim 1 2573 3. 738.448

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ULRICH J. KURZE, INVENTORS BYM ATTORNEYS SOUND SILENCING METHOD AND APPARATUS The present invention relates to acoustic muffling or silencing methods and apparatus being more particularly directed to novel structures and techniques for silencing gas turbine engines and similar noise-producing systems.

The art is replete with numerous types of soundabsorbing linings, baffles, chambers, traps and other devices for absorbing and muffling the various low and high-frequency sounds accompanying the flow of gases, such as air or exhaust in various types of enclosures, including engine exhausts, test cells, and the like. Among the techniques for absorbing the relatively lowfrequency sounds (say, 20-500 Hz, more or less), are side branches or chambers faced by acoustically absorbing flow-resistive linings, such as, for example, those described by L. Cremer in Acustica, Vol. 3, 1953, commencing at page 249, and by G. Kurtze in Acustica, Vol. 11, 1961, commencing at page 112.

In such apparatus and related devices, the airspace behind the flow-resistive lining permits the gas or other fluid medium particles carrying the acoustical energy to oscillate in the flow-resistive material and surrender part of their energy due to friction loss. It is well understood that optimal sound attenuation is achieved in narrow ducts by a relatively low flow resistance of the facing material which is less than the characteristic impedance of the gas.

For high frequencies (say, in the range of several hundred Hz to the upper end of the audio frequency range), it has been customary to employ sound absorbing material directly attached to the rigid walls of the ducts or flow paths and to interpose within the flow path space, longitudinally extending, substantially parallel baffles of sound absorbing material. Generally, the transverse spacing of such baffles is small compared with the acoustic wave lengths involved and the baffles are usually long compared with such wave lengths.

In still other instances, tuned absorbing baffles and other structures establishing both transverse and longitudinal resonance-absorption effects have been employed as described, for example, in US. Letter Pat. Nos. 2,759,554, 2,759,555, and 2,759,556.

Where both the lowand high-frequency sounds, as above defined, must be silenced, accordingly, combinations of the above-mentioned techniques have been employed; such as employing the longitudinal baffling in one section of the duct to silence the high frequencies, serially or successively followed by side branches faced with the acoustically resistant material to attenuate the low frequencies. Unfortunately, however, this composite treatment may require relatively long ducts in order to accommodate the successive silencing sections, and, in addition, does not enable synergistic or cooperative effects to take place between the silencing sections of different effective frequency range. The optimum sound attenuation of such high-frequency baffles requires, moreover, a higher flow resistance than that of the beforementioned facing material of the side branch resonators, as pointed out by Cremer, above. These parallel baffles, however, do not of themselves provide effective sound attenuation at the low frequencies.

In accordance with the present invention, on the other hand, it has been discovered that, instead of following the approach of using successive sections of silencing structures optimized for the different frequency ranges, it can be more advantageous to superimpose in the same region of the duct or enclosure both a sidebranch absorbing structure for the low-frequency sound and particularly dimensioned parallel baffles for high-frequency sound absorption. Such parallel baffles combined with side branches enable one to obtain an overall sound attenuation characteristic that is matched to the exhaust noise output of, for example, gas turbine engines and similar noise-producing systems, by emphasizing the attenuation at low frequencies. In summary, the improved low-frequency attenuation is achieved by an interaction of the lowand highfrequency sections. The mechanism of this interaction is that the reduction of wave speed due to the structure factor of the parallel baffles increases the attenuation performance of the side-branch mufflers and that an equivalent structure factor of the side branches increases the attenuation of the parallel baffles resulting in increased sound attenuation at both lowand highfrequencies.

There are, furthermore, basically two different orientations of the parallel baffles in respect to the facing of the side branches which can permit of such an interaction. First, the baffles may be disposed in a plane substantially parallel to the facing layer of the side branch. In this case, the flow resistance of the baffles should be small to permit sound transmission from the center of the duct to the side branch resonators. Secondly, the baffles may be disposed in a plane substantially perpendicular to the facing layer of the side branches. In this case, the additional low-frequency attenuation is achieved by the propagation of the low-frequency sound energy toward the side branches in the narrow channels formed by the parallel baffles. For this second configuration, the flow resistance of the baffles is not confined to small values.

In addition to the beneficial acoustical performance of the present invention, the combination of the lowand high-frequency sections may result in considerable economy of size of the required duct. It is important to point out that in applications where the gas flow carries high-intensity, low-frequency sound, and duct walls are required to have a large mass per unit area or high stiffness in order to prevent acoustic transmission; so that the savings in the necessary length dimensions of the duct achievable by the present invention may constitute a very appreciable advantage.

A further object of the invention is to provide a novel acoustic silencing structure and method of more general application as well.

Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims.

The invention will now be described with reference to the accompanying drawings,

FIG. 1 of which is a longitudinal section of an acoustical silencing duct constructed in accordance with a preferred embodiment of the invention;

FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 1 looking in the direction of the arrows; and

FIGS. 3 and 4 views, respectively, like FIGS. 1 and 2, of a modification, with FIG. 4 being a section taken along the line 4-4 of FIG. 3, looking in the direction of the arrows.

In FIG. 1, a gas or other fluid medium containing or carrying low and high-frequency sounds, such as those produced by a gas turbine, for example, is shown entering a duct housing, chamber, or enclosure I, at an inlet region 1, and passing through a novel combined low and high-frequency muffling section, emerging or exiting with the sounds attenuated at the outlet II.

The high-frequency sounds, as above defined, are primarily absorbed as the result of the use ofa plurality of longitudinally extending, substantially parallel absorbing baffles 3 (as of sound attenuation materials described in the above-mentioned articles and patents), spaced transversely at distances small compared with the sound wave lengths involved in the medium so as to provide longitudinal fluid flow paths; but the baffles are long compared to such wave lengths. Instead of lining the sides or walls of the duct I in conventional fashion, and following or preceding the absorbers 3 with a low-frequency absorbing section, the sides or walls of the duct I opposite the baffles 3 are rather provided with flow resistive layers 5 that, in turn, face side branches or cavities 7 extending transversely outward of the duct I and preferably provided with soundimpervious transverse-separating septa 9 that prevent propagation of the low-frequency sounds longitudinally along the cavity and force substantially transverse particle oscillation. In accordance with the present invention, because of the novel location of the lowfrequency section 5-7, juxtaposed to the highfrequency sound energy propagating longitudinally along the duct I, but to absorb, also, low-frequency energy forced to resonate transversely between the cavities 7, as before described. There is thus a new and supplementary effective use of the absorptive material of the longitudinal baffles 3 in aiding the low-frequency absorption produced by the low-frequency sections 5-7, which does not occur when similar devices are used serially in successive duct sections, as in the prior art.

In order to take advantage of this phenomenon, moreover, the longitudinally oriented baffle 3 must have some transparency to the acoustic energy; again as distinguished from those prior art baffles that have high flow resistance or impervious separators, septa and similar structures.

To illustrate the efficacy of the invention and the unusual and novel cooperative effects that take place by this different combination of elements, in a silencer employing three longitudinal baffles 3, substantially equally spaced across the duct I with about 30-50 percent open area, so as to provide transverse fluid flow paths, and with cavities 7 having flow resistive facing treatment 5 of substantially the same length as the co extensive baffles 3, it was found that said low frequencies are attenuated up to about 5 dB more than can be obtained with the same baffle section 3 followed by a low-frequency cavity structure 5-7 in a subsequent duct region. In the high-frequency range, no impairment in the high-frequency absorption over what would normally be produced by the longitudinal baffles 3 used alone with lined duct walls was detected. Thus, not only was improved acoustic performance obtained, particularly in the low-frequency end, but the duct treatment length required to accommodate the silencing treatment was reduced to half the size that would have been required with successive high-frequency and lowfrequency sections.

The high-frequency performance of the silencer with low-flow resistance baffles oriented substantially parallel to the facing of the side branch, as shown in FIG. 1, is somewhat reduced in comparison to a system with high-flow resistance baffles alone. This, however, does not impair the overall performance of the silencer for typical gas turbine engines and other noises producing systems with similar spectra, because standard parallel baffle silencers usually provide unbalanced attenuation in favor of higher frequencies. In those special cases where stronger high-frequency attenuation is required, on the other hand, the configuration shown in FIGS. 3 and 4 may be more advantageously employed. Here the flow resistance of the parallel baffles 3, which are this time oriented substantially perpendicular to the cavity facing 5, can be optimized so that with the given spacing a balanced highand lowfrequency attenuation is achieved.

With both the configurations of FIGS. 1 and 2 and of I FIGS. 3 and 4, moreover, not only can improved acoustical performance be obtained, particularly in the lowfrequency end, but the duct length required to accommodate the silencing treatment may be reduced. It should also be noted that the advantage of low pressure drop and self-generated noise which accompany the use of straight-through successive serially disposed silencing sections are still maintained in this unitary combined section.

While the structure has been illustrated in terms of preferred geometries, furthermore, it is to be understood that modifications in the shape, dimensioning and orientation of the baffles and other structural elements may be made, as is well-known in the art, without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. Apparatus for attenuating sound of relatively low and high frequencies accompanying the flow of a fluid medium, having, in combination, longitudinally extending duct means provided with an inlet for receiving the said fluid medium and an outlet for exiting the same, a plurality of substantially parallel longitudinal soundabsorbing baffle means extending along the duct means between the inlet and outlet and transversely spaced from one another between the sides of the duct means so as to provide a plurality of longitudinal fluid flow paths between the inlet and the outlet, the said duct means sides opening into transverse side branches extending transversely outward and longitudinally along the duct means and defining cavity means faced with flow resistive layer means and communicating through the same with the duct means, thecavities having terminating walls spaced from said layer means, and said duct means having transverse fluid flow paths between said sides.

2. Apparatus as claimed in claim 1 and in which the flow resistive layer means and the baffle means are substantially co-extensive and the said cavity means contain sound-impervious septum means for preventing low-frequency sound propagation longitudinally along the cavity means.

3. Apparatus as claimed in claim 1 and in which the transverse spacing between the baffle means and the longitudinal length of said baffle means are respectively small and large compared to the wave lengths of said sound frequencies.

4. Apparatus as claimed in claim I and in which the said substantially parallel baffle means are disposed fluid medium longitudinally along a predetermined duct region, transversely dividing said region into a plurality of longitudinally extending sub-regions, absorbing the high frequencies along a plurality of sides of each of said sub-regions, and forcing the low frequencies to travel transversely across said region and transversely outwardly therefrom while resonantly absorbing the same during such travel. 

1. Apparatus for attenuating sound of relatively low and high frequencies accompanying the flow of a fluid medium, having, in combination, longitudinally extending duct means provided with an inlet for receiving the said fluid medium and an outlet for exiting the same, a plurality of substantially parallel longitudinal sound-absorbing baffle means extending along the duct means between the inlet and outlet and transversely spaced from one another between the sides of the duct means so as to provide a plurality of longitudinal fluid flow paths between the inlet and the outlet, the said duct means sides opening into transverse side branches extending transversely outward and longitudinally along the duct means and defining cavity means faced with flow resistive layer means and communicating through the same with the duct means, the cavities having terminating walls spaced from said layer means, and said duct means having transverse fluid flow paths between said sides.
 2. Apparatus as claimed in claim 1 and in which the flow resistive layer means and the baffle means are substantially co-extensive and the said cavity means contain sound-impervious septum means for preventing low-frequency sound propagation longitudinally along the cavity means.
 3. Apparatus as claimed in claim 1 and in which the transverse spacing between the baffle means and the longitudinal length of said baffle means are respectively small and large compared to the wave lengths of said sound frequencies.
 4. Apparatus as claimed in claim 1 and in which the said substantially parallel baffle means are disposed substantially parallel to the said side branch flow resistive layer means.
 5. Apparatus as claimed in claim 1 and in which the said substantially parallel baffle means are disposed substantially perpendicular to the said side branch flow resistive layer means.
 6. A method of attenuating sound of relatively low and high frequencies accompanying the flow of a fluid medium, that comprises flowing the sound-carrying fluid medium longitudinally along a predetermined duct region, transversely dividing said region into a plurality of longitudinally extending sub-regions, absorbing the high frequencies along a plurality of sides of each of said sub-regions, and forcing the low frequencies to travel transversely across said region and transversely outwardly therefrom while resonantly absorbing the same during such travel. 